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diff --git a/Utility/SafeOutput.hs b/Utility/SafeOutput.hs
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+{- Safe output to the terminal of possibly attacker-controlled strings,
+ - avoiding displaying control characters.
+ -
+ - Copyright 2023 Joey Hess <id@joeyh.name>
+ -
+ - License: BSD-2-clause
+ -}
+
+{-# LANGUAGE TypeSynonymInstances, FlexibleInstances, CPP #-}
+{-# OPTIONS_GHC -fno-warn-tabs #-}
+
+module Utility.SafeOutput (
+	safeOutput,
+	safeOutputChar,
+) where
+
+import Data.Char
+import qualified Data.ByteString as S
+
+class SafeOutputtable t where
+	safeOutput :: t -> t
+
+instance SafeOutputtable String where
+	safeOutput = filter safeOutputChar
+
+instance SafeOutputtable S.ByteString where
+	safeOutput = S.filter (safeOutputChar . chr . fromIntegral)
+
+safeOutputChar :: Char -> Bool
+safeOutputChar c
+	| not (isControl c) = True
+	| c == '\n' = True
+	| c == '\t' = True
+	| c == '\DEL' = False
+	| ord c > 31 = True
+	| otherwise = False